1. Field of the Invention
The present invention relates to a mixture of long chain dibasic acids, a composition for an electrolytic solution for electrolytic capacitors using thereof, and the electrolytic solution for electrolytic capacitors.
2. Description of the Related Art
As a conventional electrolytic solution for electrolytic capacitors, especially as a middle and high voltage electrolytic solution for electrolytic capacitors, a solution containing ethylene glycol as a solvent and boric acid or its ammonium salt as a electrolyte is used. However, this type of electrolytic solution has problems in that the water content becomes high; that is, water is easily eliminated from the boric acid to form metaboric acid, and furthermore, water is formed from the esterification reaction between the boric acid and ethylene glycol. Thus, the water content of the electrolytic solution becomes high. As a result, when the electrolytic solution is used at a temperature over 100.degree. C., the water in the electrolytic solution is vaporized, thereby causing an increase in the inner pressure of the electrolytic capacitor which destroys the capacitor.
In order to remedy the aforementioned problems, electrolytic solutions containing saturated linear dicarboxylic acids such as azelaic acid, sebacic acid, dodecanedioic acid or the salts thereof as a electrolyte have been used. However, since the solubility of the saturated linear dicarboxylic acid to a solvent, such as ethylene glycol, is low, the saturated linear dicarboxylic acid has a tendency to be crystallized at low-temperatures. By the crystallization, excess electric current may be caused, thereby deteriorating the low-temperature characteristics. Thus, the above mentioned problems are not solved yet.
Moreover, experiments using a dibasic acid as an electrolyte have been performed. As the dibasic acid, 2-butyloctanedioic acid (Japanese laid-open patent publication No. 60-13293), 8-vinyl-10-octadecenedioic acid (Japanese laid-open patent publication No. 4-186713) and 2-methylnonanedioic acid (Japanese laid-open patent publication No. 2-224217) or salts thereof are disclosed. These dibasic acids or salts thereof are useful since the solubility of these dibasic acids or salts thereof to a solvent, such as ethylene glycol, is improved compared to the aforementioned saturated linear dibasic acids or salts thereof.
Among the dibasic acids, 2-methylnonanedioic acid is a useful long chain dibasic acid. This dibasic acid can be produced by dicarboxylation of 1,7-octadiene in the presence of a palladium-phosphine catalyst (U.S. Pat. No. 4,629,807). However, according to this method, this dibasic acid can be obtained in a yield of only 19.1%. Therefore, by this method, an electrolytic solution having the desired ability cannot be produced.
Moreover, even if 2-methylnonanedioic acid could be fully supplied, the low-temperature characteristics may not be improved because the problems of the crystallization of the electrolyte at low temperatures are not yet solved. Therefore, improvement on the low temperature characteristics is desired.
Therefore, an electrolytic solution for electrolytic capacitors which comprises a long chain dibasic acid having an alkyl group at its side chain (e.g., 2-methylnonanedioic acid) as a main electrolyte but which does not cause any crystallization of the electrolyte is desired.
In view of the above, the present inventors have conducted extensive studies on the improvement of the low-temperature characteristics of the electrolytic solution using 2-methylnonanedioic acid. As a result, the present inventors have found that when one or more long chain dibasic acid having a carbon chain of 8 or more carbon atoms between both its carboxyl groups and having at its side chain an alkyl group and one or more alkoxycarbonyl group is mixed with 2-methylnonanedioic acid, the mixture can be a desirable electrolytic solution for electrolytic capacitors in that crystallization of the electrolyte hardly occurs at low temperatures; electrical conductivity is not reduced even at high temperatures; and high-voltage resistance can be improved. Thus, the present inventors accomplished the present invention.